1. Field of the Invention
The present invention relates to a sample retainer for X-ray fluorescence analysis, which is used in pretreating a liquid sample and then in X-ray fluorescence analysis of contents of such liquid sample, an X-ray fluorescence analyzing method of utilizing such sample retainer and an X-ray fluorescence spectrometer therefore.
2. Description of the Prior Art
Hitherto, as a technique to perform X-ray fluorescence analysis for pretreating a liquid sample and analyzing contents contained in such liquid sample, a filter paper drop method (micro-droplet method) is known, in which the liquid sample is dispensed dropwise onto and dried on a filter paper so that the liquid sample can be not only concentrated, but also retained on such filter paper. However, since the filter paper has a thickness of a few hundred micrometer, scattered X-rays of the primary X-rays occurs considerable, resulting in increase of the background. Also, considering the liquid absorption capacity of the filter paper, only 50 to 100 microliter of the liquid sample can be dispensed dropwise at a time and, in the case of a trace quantity of contents, the intensity of fluorescent X-rays which will be emitted from contents concentrated on the filter paper and will subsequently be sensed by a detector may not be insufficient even though dropwise dispense and drying are repeated a number of times before the filter paper is excessively deformed. In other words, the gradient of the calibration curve (a constant associated with the fluorescent X-ray intensity, which is used in the equation of the calibration curve indicative of the concentration of the contents in the liquid sample) does not decrease sufficiently. Accordingly, the limit of detection (LLD) shown in the following equations is, for example, about a few hundred ppb in a region of heavy elements required in environmental analysis and cannot be regarded sufficient.LLD=3×b×σBG  (1)σBG=(IBG/(1000×t))1/2  (2)wherein b represents the gradient of the calibration curve, IBG represents the intensity (kcps) of the background X-rays and t represents the length of measurement time (second).
Accordingly, in order to improve the limit of detection within the bounds of the limited length of measurement time and, also, within the bounds of constant applied voltage and current, there are two ways available, in which the contents of the liquid sample are concentrated so that the sensitivity can increased to thereby improve the gradient of the calibration curve (i.e., to minimize the value of the gradient of the calibration curve. In other words, the contents are concentrated as much as possible so that the fluorescent X-rays emitted therefrom and subsequently sensed by the detector can be increased.) and in which the intensity of the background X-rays is minimized, respectively.
In order to improve the limit of detection based on these, there is a technique in which a polymer film of about 0.5 μm in thickness is formed with a vapor deposited film of, for example, carbon so that a liquid sample can be dispensed dropwise onto and subsequently dried on the area of the polymer film, where the vapor deposited film has been formed, so that contents of the liquid sample can be retained thereon. (See the Japanese Laid-open Patent Publication No. 2003-90810.)
However, since the vapor deposited film is extremely thin and has its diameter limited to about 2 mm in order for the liquid sample to be uniformly concentrated, the amount of the liquid sample that can be dispensed dropwise at a time is equal to or smaller than the amount that can be dispensed dropwise onto the filter paper. Accordingly, although the background can be reduced by the utilization of the polymer film and the vapor deposited film smaller in thickness than the filter paper, the intensity of the fluorescent X-rays that can be obtained does not increase and, therefore, improvement of the limit of detection is not sufficient. Also, with a small surface area of the vapor deposited film, even though dropwise dispensing and drying are repeated to concentrate a large amount of the contents, there are possibilities that the liquid sample cannot be retained stably and that the background will increase as a result of emission of a large amount of scattered X-rays as a result of crystallization of the contents. It is to be noted that if the surface area of the vapor deposited film is increased in order to increase the amount to be dispensed dropwise so that the intensity of the fluorescent X-rays can be increased, concentration will become uneven and emission of the fluorescent X-rays will then become uneven and unstable. (Paragraph 0019 in the previously mentioned publication.)